There are many known types of replacement heart valves. The selection of a particular type of replacement heart valve depends on factors such as the location of the valve, the age and other specifics of the patient, and the surgeon's experiences and preferences. Commonly used replacement heart valves can be classified in the following three groups: mechanical valves; allograft tissue valves; and xenograft tissue valves.
Mechanical heart valves, including, for example and without limitation, caged-ball valves, bi-leaflet valves, and tilting disk valves are typically attached to a sewing ring so that the valve prosthesis can be sutured to the patient's native tissue to hold the mechanical valve in place postoperatively. Although mechanical heart valves have advantageous long-term durability, these mechanical valves also have a propensity to cause the formation of blood clots in a patient. If such blood clots form on the mechanical valve, they may preclude the valve from opening or closing correctly or, more importantly, may disengage from the valve and embolize to the brain, causing an embolic stroke. Thus, the patients who receive such mechanical valves are typically required to take systemic anticoagulant drugs for the rest of their lives. In addition to being expensive, these anticoagulant drugs can themselves be dangerous in that they can cause abnormal bleeding in the patient that can lead to a hemorrhagic stroke.
Allograft tissue valves are harvested from human sources, such as human cadavers. Unlike mechanical heart valves, allograft tissue valves typically do not promote blood clot formation and, therefore, avoid the need for prescribing an anticoagulant medication for the patient. However, allograft tissue valves are not available in sufficient numbers to satisfy the needs of all patients who need new heart valves. Furthermore, there have been significant complications when allograft tissue valves have been used to replace atrioventricular (AV) valves within a subject. Moreover, allograft tissue valves can be more difficult to implant than mechanical valves or xenograft valves. Because of these difficulties in implantation, the operative risk associated with allograft tissue valves is often greater than the operative risks associated with mechanical valves and xenograft valves.
Xenograft tissue valves are formed from non-human tissue sources, such as cows or pigs. Most known xenograft tissue valves are constructed by sewing and/or constructing valve leaflets from a non-human tissue source and then securing the leaflets within a patient's heart using a stent and/or a sewing ring. These xenograft tissue valves are less likely to cause blood clot formation than comparable mechanical valves, and therefore, patients that receive xenograft tissue valves are not always required to take anticoagulant medications. However, xenograft tissue valves are prone to calcification and lack the long-term durability of mechanical valves and, consequently, require frequent replacement as compared to mechanical valves. One factor that may contribute to these failures is the chemical treatment that the xenograft tissue valves typically undergo to reduce antigenicity of the animal tissue. Without these chemical treatments, xenograft tissue valves can trigger an immune response in a patient, which can lead to rejection of the tissue valve by the patient. Another factor that may contribute to the lack of durability of the xenograft tissue valves is the presence of a stent and/or sewing ring, which can prevent the xenograft tissue valve from accurately approximating the anatomy of a normal heart valve.
Known tissue conduits, including those described in U.S. Pat. Nos. 5,480,424 and 5,713,950, both of which are expressly incorporated herein by reference in their entirety, suffer from various limitations, including many of the limitations of known xenograft tissue valves. For example, known tissue conduits suffer from antigenicity of the conduits, which is typically addressed using chemical treatments that lessen post-implantation durability of the conduit. Additionally, these known conduits are rapidly degraded within a patient's heart such that they can only serve as competent heart valve replacements for a matter of months.
Thus, what is needed in the art is a readily available, highly durable, and affordable tissue prosthesis that can be easily implanted to regenerate an anatomically accurate AV valve within the heart of a subject. There is a further need in the art for a sterile, acellular tissue prosthesis that can be implanted to regenerate an AV valve within the heart of a subject.